The present invention relates to the field of laser processing or ablation of materials, and is of application in, for example, the field of laser procedures for the refractive correction of the eye, in operations such as photorefractive keratectomy (PRK) and laser in-situ keratomileusis (LASIK).
Refractive surgery is currently performed with the excimer laser, which operates at a wavelength of 193 nm. A laser delivery system is used in these procedures to control the shape that is etched onto the cornea. The ablated shape can be controlled by a variety of methods, including the use of a large beam, or a scanning beam, combined with masks, templates or diaphragms (see, for example, U.S. Pat. No. 5,474,549). Other systems include apparatus that scan the beam across the area to be ablated in a predetermined pattern.
Large beam control system using masks and diaphragms require large, high energy laser sources, which entail increased running and maintenance costs. These systems are also inflexible and limited in their ability to produce complicated shapes on the material to be ablated. In comparison, scanning methods require smaller, lower energy laser sources, making them more space and cost efficient. It is also easier to control the desired shape of the ablation using a scanning method (Ren, Simon and Parel, 1993).
Different mechanisms for scanning the beam across the surface to be ablated have been suggested in a number of prior art patents. U.S. Pat. No. 4,718,418 teaches the use of a commercially available scanning unit to scan a rounded-square spot of 0.5 mm by 0.5 mm across the surface to be scanned. The inventors suggest that no overlap should occur between individual pulses, with the areas of greater tissue removal, such as the central cornea in a myopic correction, to be scanned with the square dot more often than those areas with less material to be removed.
U.S. Pat. No. 5,520,679 describes a scanning method using a low power, high repetition rate laser. Uniform beam density or a specific spot shape is not required. The galvanometer scanning device is coupled with a computer controller, and synchronized with the laser""s repetition rate to move in predetermined patterns. A mathematical model for optimum beam overlap is provided, indicating that ablations should overlap between 50% and 80% to avoid a ridged corneal surface.
As described above, electrical galvanometer scanners have been used as mechanisms for scanning the laser beam in prior art devices. These scanning systems utilise mirrors mounted on galvanometer apparatus, which thereby produce a motion to move the mirrors to scan the beam in a predetermined pattern in X or Y or X-Y directions. However, a number of disadvantages are associated with galvanometer scanning systems. The amount of light reflected by the mirrors in such systems is dependent on the angle of these mirrors. Changing the angle of the galvanometer mounted mirrors may alter the energy of the laser beam delivered to the cornea. The beam path length may also be affected by the galvanometer mirrors. Any change to the path length may subsequently result in a loss of beam focus. If the distance from the scanner to the eye is not well controlled, then the position of the laser as it hits the eye cannot be properly predicted and the resultant surgery will be inaccurate.
It is therefore an object of the present invention to provide an improved scanning laser beam control system that can overcome one or more of the limitations of the prior art scanning systems and more accurately and predictably ablate a desired shape into a material.
It is a further object of the present invention to provide a scanning system that can be more easily fitted to non-scanning laser systems.
Thus, according to the present invention there is provided a laser scanning method for scanning material with a laser beam including directing a laser beam through a scanning means, wherein said scanning means includes a first lens and a second lens, the two lenses separated by a distance approximately equal to the focal length of one of the lenses, and said method includes directing said beam through said lenses in turn to said material, and controlling said beam by moving at least one of said lenses in a plane perpendicular to said beam to scan said material with said beam.
Preferably said method includes passing said beam through said second lens after said first lens, and the two lenses are separated by a distance approximately equal to the focal length of said second lens.
Preferably the method includes providing said laser beam by means of an Argon-Fluoride excimer laser (193 nm) or a solid state UV laser (190-215 nm) such as quintupled Nd:YAG lasers, or infra-red lasers such as Ho:YAG or Er:YAG lasers.
Preferably the method includes controlling the scanning means to scan the laser beam, and more preferably in predetermined patterns.
Preferably the method includes controlling said scanning means by means of computer means.
The first lens may have a focal length substantially longer than the second lens.
The present invention also provides a scanning apparatus for scanning material with a laser beam including laser means for producing a laser beam, scanning means for scanning the laser beam in a predetermined pattern onto the material; and wherein said scanning means includes two lenses, separated by a distance approximately equal to the focal length of one of the lenses, and at least one of said first and second lenses is mounted for movement perpendicular to said beam to scan said beam.
Preferably said first and second lenses are arranged so that said beam passes through said second lens after said first lens, and the two lenses are separated by a distance approximately equal to the focal length of said second lens. The first lens is preferably mounted for movement perpendicular to the laser beam.
Preferably the laser means is an ablation laser, and more preferably the laser means is an UV ablation laser such as an Argon fluoride excimer laser, quintupled Nd:YAG, or a quadrupled Ti:Sapphire laser, or an infra-red ablation laser such as Er:YAG, or an intrastromal ablation laser such as a Ho:YAG, Nd:YAG or Nd:YLF laser. For example, for intrastromal, or lens ablation, the laser means would preferably be a visible or near infra-red laser such as Nd:YAG or Nd:YLF.
Preferably the scanning means is controllable to scan said laser beam, and more preferably to scan said laser beam in predetermined patterns.
Preferably the scanning means is controllable to scan said beam to follow, or compensate for, movements of said material.
Thus, the scanning means can scan the beam to compensate for movement of the material.
Preferably the controlling means includes a microprocessor means or a computer means.
Preferably the apparatus is for scanning a laser across the cornea of an eye in surgical procedures such as PRK, LASIK, intrastromal ablation, or across the lens in a phaco-emulsification procedure.
One of the two lenses may have a focal length substantially longer than the other.
Preferably the apparatus is for scanning skin with any one of various laser beams by directing any one of said laser beams through said scanning means.
The present invention further provides a method for ablating human or animal tissue including directing a laser beam through a scanning means onto an area of said tissue to thereby ablate said tissue, wherein said scanning means includes two lenses, separated by a distance equal to the focal length of one of the lenses, and said method includes controlling said beam by moving at least one of said lenses in a plane perpendicular to said beam to scan said material with said beam.
Preferably said tissue is corneal tissue.
Alternatively said tissue is lens tissue and said method is for breaking up a lens prior to cataract surgery.
Preferably said method is used to correct refractive errors of eyesight, by PRK, LASIK or intrastromal ablation.
Preferably the scanning means is controllable to scan the laser beam.
Preferably the scanning means is controllable to scan the laser beam in predetermined patterns and/or follow movements of said material.
In one preferred embodiment, there is provided a laser scanning method for scanning skin with various laser beams by directing said laser beams through said scanning means.
In another preferred embodiment, there is provided a scanning apparatus for scanning skin with various laser beams by directing said laser beams through said scanning means.
Preferably said laser beams are provided by any laser used for dermatological uses, hair removal or photodynamic therapy.
Preferably the first lens is mounted in a mount and said mount is attached to a static mount by means of two pairs of linear bearings or slides, and wherein said pairs of bearings or slides are either arranged with respect to each other, or attached to each other, at right angles.